Mechanical power transmission systems are often built from several connected modules. Such modules can be an electric motor and a mechanical reducer, or a steam turbine and an electric generator, etc. The shafts of these modules are connected with couplings which are capable to transmit torque and rotational motion (torsional connections). In some cases, these torsional connections are rigidly attached to both connected shafts, and also have rigid structures ("rigid couplings"). However, if there are misalignments between the connected shafts (offset, angular misalignment, etc), use of rigid couplings results in undesirable alternating loads of significant magnitudes applied to both connected shafts and to their bearings. Since the misalignments are practically always present due to inaccuracies of assembling the modules, due to their different thermal expansion, due to deformations of the modules and their mounting systems under transmitted loads, due to sagging of foundations, etc., there is a need for torsional connections which are rigid in torsional direction (for torque transmission), but have a degree of mobility in the directions of possible misalignments between the connected shafts, from which an offset is often the most important one.
While widely used torsionally-flexible couplings (such as jaw coupling, tire coupling, etc.) always have some degree of misalignment-compensation ability, in many cases their use is restricted since high torsional stiffness of the connection is required. Gear couplings have high torsional stiffness, but are able to compensate only angular misalignments, thus two gear couplings connected by a rather long spacer are required for compensating offset misalignments. This approach, while frequently embraced, is expensive and requires a substantial space.
A very compact torsional connection which provides kinematic compensation for offset misalignment is Oldham coupling which consists of two hubs attached to the connected shafts and an intermediate member which is in a sliding engagement with these hubs. The intermediate member has projections on each face, which are located on each face along the same diameter. These diameters on two faces are perpendicular to each other. The projections have sliding engagements (on two sides of each projection) with corresponding slots on each hub. During rotation, the intermediate member slides in relation to both hubs, thus providing a theoretically ideal compensation of the offset misalignment between the connected shafts. In real life, performance of conventional Oldham couplings is far from ideal. Since the projections are sliding in their respective slots while loaded with high tangential forces due to torque transmission by the connection, high loads (friction forces) are still transmitted to the connected shafts and their bearings. Friction coefficients are high due to small dimensions of the sliding connections and ensuing high contact pressures, difficulties to supply lubrication, inevitable distortions of the contact conditions, etc. At small offset misalignments, compensation does not occur since the forces transmitted to the shafts do not exceed static friction forces and thus sliding does not commence. Oldham couplings are limited to low-speed applications due to the bad friction conditions. They have significant power transmission losses and, thus, heat generation, which forces designer to provide for clearances in the projection--slot sliding pairs. The resulting backlashes make such couplings (as well as gear couplings) very undesirable for applications in servo-controlled systems in which presence of backlashes hampers realization of the required control strategies. Also, due to presence of sliding at high contact pressures, conventional Oldham couplings have relatively large dimensions.
Thus, the prior art does not satisfy a need for a compact torsionally--rigid misalignment--compensating torsional connection (coupling) which can be used at high rpm, generate low loads on connected shafts and their bearings, perform at any, even very small., misalignments, and be free from backlash.
The present invention addresses the inadequacies of the prior art by providing a torsional connection which retains the kinematic structure of the Oldham coupling but does not have its design shortcomings. The proposed torsional connection is composed of two hubs, attached to the respective shafts, and a coaxial intermediate member connected with the hubs as in the Oldham coupling. However, instead of sliding connections between the hubs and the intermediate member which are characteristic for the conventional Oldham coupling, each sliding contact is replaced with a laminated element consisting of bonded together alternating, preferably thin, layers of elastomeric (rubber-like) and rigid (e.g., metal or composite) material which are accommodating tangential forces due to torque transmitted between the hubs and the intermediate member, in compression (i.e., in normal direction to the layers). The displacements between the hubs and the intermediate member which are compensating offset misalignments between the connected shafts are materializing through internal shear deformations in the laminated elements (instead of the sliding motions as in the conventional Oldham coupling). Since it is known (e.g., see E. Rivin, "Properties and Prospective Applications of Ultra-Thin Layered Rubber-Metal Laminates for Limited Travel Bearings", Tribology International, February 1983) that shear resistance of the laminated elements is only slightly dependent on its compression loading, and that compression stiffness of the laminated elements is fast increasing with increasing compression loading, the laminated elements in the present invention are preloaded in compression during assembly of the torsional connection. The preload is applied by means of elastic deformation of the laminate--holding structures, thus simplifying the assembly procedure. Each of the fork-like holding structures has two prongs contacting with the laminated elements. The prongs are simultaneously deflected outwardly by a preload--generating element moving inside the holding structure. After preloading is accomplished, the holding structure remains reinforced by the preload--generating element into a rigid and strong frame-like structure which then participates in torque transmission through the connection.
The preload allows to completely eliminate clearances/backlashes in the torsional connection, and to significantly increase its torsional stiffness. These effects are achieved without a significant increase (or even with reduction) of the forces required to provide the misalignment--compensating effect and, accordingly, without increase in dissipation of the transmitted energy in the torsional connection.
Use of internal shear deformation instead of sliding friction makes the torsional connection sensitive to misalignment of any magnitude, even very small. Since no heavily loaded frictional contacts are used, material specifications to major structural components of the torsional connection (the hubs and the intermediate member) can be relaxed since only their bulk strength and not contact endurance is required. Accordingly, light and strong advanced materials (such as aluminum, fiber-reinforced composites, etc) can be used for the torsional connection, thus reducing its weight and centrifugal forces at high rpm. It was also shown (e.g., in the cited above article) that the laminated elements can tolerate much higher specific loads for limited travel applications than the sliding contacts. This results in a smaller size of the torsional connection described in the present invention than the size of a conventional Oldham coupling for the same payload capacity.